User account details are now accessible

While developing an addon, it become apparent that we are missing the ability obtain the authenticated user's information, so I added it in.

This is purely read-only at the moment, but it serves as a good starting point.

```
>>> import heroku
>>> code = "redacted"
>>> c = heroku.from_key(code)
>>> c.account
<account 'dude@heroku.com'>
>>> c.account.email
u'dude@heroku.com'
>>> c.account.beta
>>> c.account.verified
True
>>> c.account.allow_tracking
True
>>> c.account.id
u'1995845@users.heroku.com'
>>> c.account.created_at
datetime.datetime(2013, 3, 19, 15, 12, 28, tzinfo=tzoffset(None, -25200))
>>> c.account.updated_at
datetime.datetime(2013, 7, 16, 20, 29, 39, tzinfo=tzoffset(None, -25200))
>>> c.account.last_login
datetime.datetime(2013, 7, 16, 20, 29, 39, tzinfo=tzoffset(None, -25200))
```